No deadass there's no way someone didnt think of that before. And I'm pretty sure someone would have thought to use wax if they couldnt use magic.
We have the technology to: take people to space; use pig organs in transplants; clone animals; GPS; the cluster-fudge that is AI etc.
Even with all of that:
If YOU get water on a page or book it'll get damp if not ruined, right?
Often an invention seems obvious only once the thing or idea is invented
Ig most people just dont need waterproof paper. In our age, if we need to preserve paper, we can laminate it. Most of the time people just use online documents and print everything so even if it does get ruined, it can be reprinted so having water proof paper would just be useless and a waste of money for us. I mean even if it was waterproof, that doesnt make it fireproof or rip proof so in the end, it has no real use besides saving some effort or preventing it from weathering.
Its not that it doesn't exist, but mc is planning to use disposable paper cups in her Armand store not the spell itself is her invention
Wait that makes so much more sense
FL: Let’s use magic to make waterproof paper
Girl: Oh my God! You're a genius!!!
Everyone before: Hey we have this magic that can preserve things can we use it to make waterproof paper, a prevalent problem that has plagued many for centuries?
Someone else: No you need to wait for the protagonist to invent that
Everyone before: What the f*ck does that mean?!